1. Field of the Invention
This invention relates to a power source unit in which a plurality of rapidly chargeable and dischargeable batteries having low internal resistance are connected in series, and an electric current flowing in the batteries is automatically limited when the temperature in the batteries of the unit is increased.
2. Description of the Prior Art
There has been developed in recent years a power source unit in which a plurality of rapidly chargeable and dischargeable batteries having low internal resistance for example, nickel-cadmium batteries are connected in series and assembled and packed with insulating materials to form an integral battery assembly. The power source unit has low internal resistance and can be charged with a high charging current considering its small volume and can discharge a high discharging current. Moreover, the power unit can be charged normally in a few minutes. These advantages, therefore, are made full use of in various electric-operated toys, for example, radio-controlled airplanes.
However, when charging or discharging the power source unit, an extremely large current may sometimes flow in the power source unit. In such a case, the temperature of the batteries is unwantedly increased, and along with this, the internal pressure of the batteries is also increased. To cope with this problem, various measures, such as a safety valve, have been devised to prevent an increase in the internal pressure of the batteries. In the case of a safety valve, however, gas in the batteries is released to the outside as the safety valve is operated, resulting in a deterioration in the performance of the batteries or reduction in the life of the batteries.